Why do physicists care so much about finding the Higgs boson?

A particle, which might be a Higgs boson, decaying into muons in the ATLAS detector. Credit: ATLAS Experiment/CERN

If you've read anything about the Higgs boson, you probably know that this particle is special because it can explain how fundamental particles acquire mass. Specifically, evidence of the boson is evidence that an omnipresent Higgs field exists—one that slows particles down and makes them heavy.

But there's a misconception that sometimes creeps into this explanation. The Higgs field does notexplain the origin of all mass. "Many uninformed physicists have been saying that for years," says theoretical physicist Chris Quigg of Fermi National Accelerator Laboratory.

"We have actually understood the source of most of the mass in the proton [for example] for some time," Quigg says. Most mass—including your own—comes from the strong force, a force of nature that keeps the nucleus of atoms bound together.

If you're concerned that you've been hoodwinked into celebrating a particle with slight implications, fear not. "It's actually much more exciting than that, and when you hear the whole story, the symmetries and laws, it's more amazing to think that this works," says Columbia University physicist Tim Andeen. There's a more complex question involved, one that has grander implications for the way the forces of nature work. The Higgs mass-giving mechanism is key to explaining a mystery called "spontaneous electroweak symmetry breaking." Before your eyes glaze over, let's break down that vocabulary a bit.

When you think of symmetry, you probably think of beautiful faces, Classical architecture, and an Art 101 course on drawing. Physicists think of symmetry in terms of sameness. For example, because we can do an experiment in two different places and get the same result, we know that the universe is spatially symmetric. The laws of physics don’t change with time, either, which means the universe is temporally symmetric. These symmetries are inextricably linked to the laws of the universe, such as the conservation of momentum and energy. If you hit upon a symmetry, there must be a law of conservation accompanying it—and vice-versa.

“Spontaneous,” in this case, implies a certain amount of random chance—nothing has been predetermined. Imagine going to a dinner party and sitting down at a round table, only to realize that no one is sure whether they should take the bread roll to the left or bread roll to the right. At some point, someone will just grab a roll—spontaneously breaking the symmetry of the place settings.

Electroweak refers to two forces of nature: the electromagnetic force, which unites electricity and magnetism, and the weak force, which governs radioactive decay. Why have these two forces been squelched together? Physicists have long been attempting to unify the forces of nature, both because a single description is more simple and elegant, and because at very high energies, these forces appear to become one single force.

In the 1960s, many physicists were working on reconciling the electromagnetic and weak forces into a unified theory. Three physicists in particular—Sheldon Lee Glashow, Abdus Salam, and Steven Weinberg—developed something called “electroweak theory,” which neatly extended our understanding of electromagnetism to incorporate the weak force.

Yet the theorists hit a snag. In order to combine these forces, they needed to introduce a set of force-carrying particles for the weak force to complement the photon—the mass-less particle that carries the electromagnetic force. For the electromagnetic and weak forces to unify, their force carriers would have to be symmetric. In consequence, none of them should have any mass.

But it turned out that the weak force–carrying particles (the W and Z bosons) did have mass. In fact, they were quite heavy. The Higgs mechanism (which had been proposed and developed by several theorists in addition to Peter Higgs) offers the solution. It suggests that when the Higgs field interacts with the W and Z bosons, the Higgs field spontaneously breaks the symmetry that would have kept the W and Z massless. It masks their true massless nature. Finding the Higgs, therefore, is part of a larger quest to unify the forces of nature, with implications that sweep across the laws that govern the universe.

There's another reason to care about the Higgs boson. Quigg has written extensively on the consequences of a Higgs-free universe. He points out that when Salam and Weinberg were working out how the Higgs mechanism might help unify electromagnetism and the weak force, they realized that in addition to giving mass to the heavy force-carriers, the Higgs might also give mass to other fundamental particles.* The electron, therefore, would owe its mass to the Higgs field. Without that mass, electrons wouldn’t hook up with nuclei to form atoms. "That would mean no valence bonding, so much of chemistry, essentially all, would vanish," Quigg says. "Therefore no solid structures and no template for life."

*Correction (11/30/2012): This post was edited after posting to remove Sheldon Glashow from this sentence. Glashow was involved in the creation of electroweak theory but it was Salam and Weinberg who combined electroweak symmetry breaking with the Higgs mechanism.

The views expressed are those of the author and are not necessarily those of Scientific American.

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